1. Field of the Invention
The present invention relates to a planar array antenna formed of a microstrip conductor and capable of being used as a transmission/reception antenna of a radar mounted on a vehicle.
2. Description of the Related Art
U.S. Pat. No. 4,063,245 discloses a conventional planar array antenna formed of a microstrip conductor. As shown in FIG. 18, in the antenna disclosed in U.S. Pat. No. 4,063,245, a ground conductor layer 2 is formed on a reverse surface of a dielectric substrate 1, and a plurality of straight feeder microstrips 3 are formed on the dielectric substrate 1. The feeder microstrips 3 extend in parallel to each other and have first ends connected together and second ends of open-circuit termination (hereinafter referred to as xe2x80x9copen endsxe2x80x9d). A plurality of antenna elements 4a to 4e project transversely from each feeder microstrip 3 in the form of branches. Thus, a linear array is formed. The feeder microstrips 3 each forming a linear array are connected to a feeder microstrip 5, and a composite signal is output from the center 6 of the feeder strip 5. Thus, a two-dimensional array antenna is configured.
The antenna elements 4a to 4e are disposed at a pitch corresponding to the guide wavelength xcexg of electromagnetic waves that propagate within the feeder microstrip (hereinafter simply referred to as the xe2x80x9cguide wavelengthxe2x80x9d), and the length of the antenna elements 4a to 4e is set to about half the guide wavelength xcexg; i.e., xcexg/2.
Since the excitation amplitude of each of the antenna elements 4a to 4e can be controlled through a change in the width thereof, the antenna can have desired directivity-related characteristics; i.e., gain and side lobe level, which are determined in accordance with the intended use (specifications). In the illustrated example, antenna elements nearer either end of each feeder microstrip 3, such as 4a and 4e, are narrower than those nearer the center of the feeder microstrip 3, such as 4c; and the antenna element 4e is connected to the feeder microstrip 3 at a point half the guide wavelength xcexg from the open end 7 of the feeder microstrip 3. Thus, standing-wave excitation is enabled, and each linear array can have a peak-like amplitude distribution such that the amplitude increases toward the center of the feeder microstrip 3. This amplitude distribution has the effect of shrinking side lobes.
FIG. 19 is a plan view showing the structure of another conventional array antenna. This array antenna comprises a straight feeder microstrip 53 as in the above-described conventional antenna, and a plurality of antenna elements 54a to 54t projecting transversely from the feeder microstrip 53 in the form of branches. One end of the feeder microstrip 53 is connected to an input/output port 56, and the other end is connected to a matching termination element 58a, whereby traveling-wave excitation is realized. The antenna elements 54a to 54j in a first set project perpendicularly from one side of the feeder microstrip 53 at a pitch corresponding to the guide wavelength xcexg. Further, the antenna elements 54k to 54t in a second set project perpendicularly from the other side of the feeder microstrip 53 at a pitch corresponding to the guide wavelength xcexg. The positions at which the antenna elements 54a to 54j in the first set are connected to the feeder microstrip 53 are offset by xcexg/2 from the positions at which the antenna elements 54k to 54t in the second set are connected to the feeder microstrip 53.
The above-described structure makes it possible to increase the number of antenna elements within a unit path length and to reduce the residual power reaching the terminal end, which residual power lowers the efficiency of an antenna which has a relatively short array length and is excited by traveling waves. Therefore, the structure can realize an antenna which operates efficiently even when the array length is relatively short (about 10xcexg in the antenna shown in FIG. 19). Further, in the conventional antennas shown in FIGS. 18 and 19, the antenna elements 4a to 4e or the antenna elements 54a to 54t radiate electromagnetic waves mainly from their open ends and can therefore be considered to approximate magnetic dipoles. Therefore, radiated or received electromagnetic waves have a plane of polarization perpendicular to the feeder microstrip 3 or 53.
Moreover, an antenna as shown in FIG. 20 is known. In this antenna, antenna elements 74a to 74e are formed to incline with respect to a feeder strip 73 such that the antenna elements 74a, 74b, and 74c located on one side of the feeder strip 73 incline at an angle of about +45 degrees with respect to the feeder strip, and the antenna elements 74d and 74e located on the other side of the feeder strip 73 incline at an angle of about xe2x88x9245 degrees with respect to the feeder strip, whereby circularly polarized waves are produced. The antenna elements 74a and 74d are symmetrical with respect to a line Axe2x80x94A passing through the center of the feeder microstrip 73 and are disposed such that the distance between the antenna elements 74a and 74d becomes xcexg/4. In other words, an electric field Ea which is radiated from the antenna element 74a at an angle of +45 degrees relative to the feeder microstrip 73 and an electric field Ed which is radiated from the antenna element 74d at an angle of xe2x88x9245 degrees relative to the feeder microstrip 73 are composed with a phase difference of 90 degrees, so that circularly polarized waves are radiated mainly in the direction of a main beam.
Moreover, an array antenna having a structure as shown in FIGS. 21A and 21B is described in xe2x80x9cDesign of Low Cost Printed Antenna Arraysxe2x80x9d (J. P. Daniel, E. Penard, M. Nedelec, and J. P. Mutzig, Proc. ISAP, pp. 121-124, Aug. 1985). On a dielectric substrate 101 (201) are disposed 10 square microstrip antenna elements 104 (204) which are connected to a feeder microstrip 103 (203) such that power is fed to the microstrip antenna elements 104 (204) from their corners. The plurality of microstrip antenna elements 104 (204) are disposed symmetrically along the longitudinal direction with respect to an input/output terminal 102 (202) formed at the center of the feeder microstrip 103 (203). In the antenna of FIG. 21A, the microstrip antenna elements 104 are connected to one side edge of the feeder microstrip 103 at a pitch corresponding to the guide wavelength xcexg of the feeder microstrip 103, and an impedance transformer 105 having a length of xcexg/4 is formed on the upstream side (the side closer to the input/output terminal 102) of each connection point. In the antenna of FIG. 21B, the microstrip antenna elements 204 are alternately connected to opposite side edges of the feeder microstrip 203 at a pitch corresponding to half the guide wavelength xcexg of the feeder microstrip 203, and an impedance transformer 205 having a length of xcexg/4 is formed on the upstream side (the side closer to the input/output terminal 202) of each connection point.
By virtue of the above-described structure, in the antenna of FIG. 21A, degenerated TM01, and TM10, modes perpendicular to the microstrip antenna elements 104 are excited, so that an electromagnetic wave polarized in a direction perpendicular to the feeder microstrip 103 is generated as a composite polarized wave. Similarly, in the antenna of FIG. 21B, an electromagnetic wave polarized in a direction perpendicular to the feeder microstrip 203 is generated. Further, in the antennas of FIGS. 21A and 21B, through adjustment of the conversion impedance of the impedance transformers 105 and 205, the excitation amplitude of each of the microstrip antenna elements 104 and 204 can be controlled in order to attain desired directivity-related characteristics. Further, in the arrangement shown in FIG. 21B, the microstrip antenna elements 204a and 204b produce respective wave components perpendicular to the main polarized waves (polarized waves perpendicular to the feeder microstrip 203) such that the components are excited in opposite phases and are thus cancelled out. Therefore, the level of cross-polarized waves is reduced.
The above-described microstrip array antennas have the advantages of a thin shape and high productivity, and are therefore widely applied to systems used in the microwave band. Further, in the millimeter-wave band, they are applied to on-vehicle radars for collision prevention or ACC (Adaptive Cruise Control).
In the case of on-vehicle radars, waves linearly polarized at an angle of 45 degrees with respect to the ground must be used in order to avoid interference with waves radiated from a radar mounted on an oncoming vehicle. However, in a conventional antenna, since antenna elements extend vertically from a feeder line regardless of whether the antenna is of standing-wave excitation type or travelling-wave excitation type, only waves polarized in a direction perpendicular to the feeder microstrip can be generated. That is, waves polarized in a desired direction cannot be obtained. Although there has been proposed an arrangement in which antenna elements are disposed on opposite sides of a feeder microstrip such that the antenna elements incline at symmetric angles with respect to the feeder microstrip, the arrangement is adapted to generate a circularly polarized wave and cannot generate a linearly polarized wave.
In the microstrip antennas shown in FIGS. 21A and 21B, power is fed to each microstrip antenna element via a corner thereof, so that degenerated modes are generated as shown in FIG. 22A. Therefore, each microstrip antenna element operates in the same manner as an antenna element shown in FIG. 22B. Accordingly, like the case of the array antennas of FIGS. 18 and 19, only waves polarized in a direction perpendicular to the feeder microstrip can be generated. Further, in these antennas, the excitation amplitude of each microstrip antenna element is controlled by means of an impedance transformer inserted into the feeder microstrip. Therefore, when the impedance is low, the width of the feeder microstrip becomes excessively large, which hinders disposition of microstrip antenna elements. Further, when the impedance is high, the width of the feeder microstrip becomes excessively small, which renders fabrication of the antennas difficult because of limits in relation to fabrication.
The present invention was accomplished in order to solve the above-described problems, and an object of the present invention is to provide a microstrip array antenna which enables radiation and reception of waves polarized in a direction inclined with respect to a feeder microstrip.
Another object of the present invention is to provide a microstrip array antenna which has excellent reflection characteristics and high radiation efficiency.
In order to achieve the above objects, a microstrip array antenna according to a first aspect of the present invention comprises a dielectric substrate, a strip conductor formed on a top face of the dielectric substrate, and a ground plate formed on a reverse face of the dielectric substrate, wherein the strip conductor comprises a straight feeder stripline, and a plurality of radiation antenna elements disposed along at least one side of the feeder stripline at a predetermined pitch. The radiation antenna elements are connected to the feeder stripline and each have an electric field radiation edge which is not parallel to the longitudinal direction of the feeder stripline. Each of the radiation antenna elements is formed of a strip conductor having a base end connected to said feeder stripline, and an open distal end, and has a length approximately equal to an integral number times half wavelengths of electromagnetic waves which propagate along the feeder stripline at a predetermined operating frequency, and a width determined according to excitation amplitude of respective radiation antenna element, said excitation amplitude being determined so as to provide a desired directivity.
According to a second aspect of the present invention, each of radiation antenna elements has a strip-like shape, so that the width of each radiation antenna element is smaller than the length thereof.
According to a third aspect of the present invention, each of the radiation antenna elements has a rectangular shape and is connected to the feeder stripline via only a corner of the antenna element or a portion in the vicinity of the corner.
According to a fourth aspect of the present invention, the array antenna has a first region in which each of the radiation antenna elements has a comparatively narrow width and a second region in which each of the radiation antenna elements has a comparatively wide width. The radiation antenna element in the first region has a strip-like shape with a constant width and a length larger than the width and is connected to the feeder stripline via the entirety of the base-end side. The radiation antenna element in the second region has a rectangular shape and is connected to the feeder stripline via only a corner of the antenna element or a portion in the vicinity of the corner.
According to a fifth aspect of the present invention, the radiation antenna element having the strip-like shape is used in a region in which each antenna element has a width less than about 0.075 times a free-space wavelength at the operating frequency, and the radiation antenna element having the rectangular shape is used in a region in which each antenna element has a width equal to or greater than about 0.075 times the free-space wavelength at the operating frequency.
According to a sixth aspect of the present invention, the electric field radiation edge of each radiation antenna element forms an angle of about 45 degrees with respect to the feeder stripline.
According to a seventh aspect of the present invention, each of the radiation antenna elements has a rectangular shape in which the length differs from the width.
According to an eighth aspect of the present invention, each of the sides of each rectangular radiation antenna element which form the corner connected to the feeder stripline forms an angle of about 45 degrees with respect to the feeder stripline.
According to a ninth aspect of the present invention, the radiation antenna elements comprise first radiation antenna elements formed along a first side of the feeder stripline and second radiation antenna elements formed along a second side of the feeder stripline opposite the first side. The second radiation antenna elements have the same shape as that of the first radiation antenna elements and are disposed substantially in parallel to the first radiation antenna elements.
According to a tenth aspect of the present invention, the first radiation antenna elements formed along the first side of the feeder stripline radiate electric fields in a direction substantially parallel to a direction in which the second radiation antenna elements formed along the second side of the feeder stripline radiate electric fields.
According to an eleventh aspect of the present invention, each of the second radiation antenna elements is disposed at an approximately center point between adjacent first radiation antenna elements disposed along the feeder stripline.
In the microstrip array antenna according to the present invention, a plurality of radiation antenna elements are connected to at least one side of the feeder stripline at a predetermined pitch such that the electric field radiation edge of each antenna element inclines at a certain angle with respect to the longitudinal direction of the feeder stripline. Therefore, electric fields produced perpendicular to the electric field radiation edge generate electromagnetic waves polarized in a direction which is not perpendicular to the feeder stripline but which inclines with respect to the feeder stripline. Accordingly, when the microstrip array antenna is used as an antenna of a radar for automotive use, the antenna does not receive electromagnetic waves from oncoming vehicles. Further, the microstrip array antenna can have a desired directivity through a proper design in which the width of each radiation antenna element is changed in accordance with a desired excitation amplitude.
The term xe2x80x9celectric field radiation edgexe2x80x9d of the radiation antenna element means a side of the radiation antenna element perpendicular to the direction of an electric field to be radiated.
In the second aspect of the present invention, since each radiation antenna element has a strip-like shape, such that the width of each radiation antenna element is smaller than the length thereof, polarized waves of a single mode can be obtained.
In the third aspect of the present invention, each radiation antenna element has a rectangular shape and is connected to the feeder stripline via only a corner of the antenna element or a portion in the vicinity of the corner. Therefore, opposite sides of each radiation antenna element parallel to the longitudinal direction thereof have substantially the same length. This enables generation of electromagnetic waves of a single mode polarized in the longitudinal direction to thereby obtain excellent directivity while lowering the level of cross-polarized waves. Accordingly, when the microstrip array antenna is used as an antenna of a radar for automotive use, the antenna does not receive electromagnetic waves from oncoming vehicles. Further, since the reflection of each radiation antenna element is reduced, the radiation efficiency or reception sensitivity of the array antenna can be increased. Further, a desired directivity can be obtained through a design in which the width of the radiation antenna element is changed in accordance with its position on the feeder stripline.
In the fourth aspect of the present invention, each radiation antenna element has a certain shape and is connected to the feeder stripline in a certain manner, the shape and the manner of connection being determined in accordance with the width of the radiation antenna elementxe2x80x94which changes in accordance with position on the feeder stripline in order to obtain a desired directivity. Thus, there can be realized an array antenna in which reflection at each element is minimized. Therefore, it becomes possible to fabricate an array antenna having a high radiation efficiency or reception sensitivity.
In the fifth aspect of the present invention, a radiation antenna element having the strip-like shape is used in a region of the width distribution in which each antenna element has a width less than about 0.075 times a free-space wavelength at the operating frequency, and a radiation antenna element having a rectangular shape is used in a region of the width distribution in which each antenna element has a width equal to or greater than about 0.075 times the free-space wavelength at the operating frequency. Thus, each radiation antenna element has desirable reflection characteristics, which enables production of high-efficiency array antennas having different directivities.
In the sixth aspect of the present invention, since the electric field radiation edge of each radiation antenna element forms an angle of about 45 degrees with respect to the feeder stripline, the microstrip array antenna can generate electromagnetic waves which are polarized at an angle of about 45 degrees with respect to the feeder stripline. Therefore, when the microstrip array antenna is mounted on a vehicle such that the feeder stripline extends perpendicular to the ground surface and is used as an antenna of a radar, reception of electromagnetic waves from oncoming vehicles can be prevented most effectively.
In the seventh aspect of the present invention, each of the radiation antenna elements has a non-square, rectangular shape such that the length differs from the width. This structure suppresses excitation of other modes more effectively, to thereby facilitate generation of waves of a single mode.
In the eighth aspect of the present invention, each of the sides of each rectangular radiation antenna element which form the corner connected to the feeder stripline forms an angle of about 45 degrees with respect to the feeder stripline. Therefore, electromagnetic waves can be polarized at an angle of about 45 degrees with respect to the feeder stripline, so that the same effect as that obtained in the sixth aspect can be obtained.
In the ninth aspect of the present invention, since the radiation antenna elements are disposed on both sides of the feeder stripline such that all the radiation antenna elements are directed toward the same direction, the microstrip array antenna can have improved electromagnetic-wave radiation efficiency and improved reception sensitivity.
In the tenth aspect of the present invention, since the first and second radiation antenna elements have the same direction of polarization in which electromagnetic waves are polarized, the microstrip array antenna can have improved electromagnetic-wave radiation efficiency and improved reception sensitivity.
In the eleventh aspect of the present invention, since the radiation antenna elements are alternately disposed along both sides of the feeder stripline at equal intervals, the microstrip array antenna can radiate and receive electromagnetic waves with high efficiency and has improved directivity.